1. Field of the Invention
The invention relates to antenna systems in which an antenna is arranged to transmit or receive radiation through a transmissive surface.
2. Discussion of Prior Art
There are a number of situations in which an antenna lobe is directed through a transmissive surface. For example, a monolithic microwave integrated circuit (MMIC) package may comprise a MMIC chip having an antenna, the chip being contained with a ceramic package. Electromagnetic (EM) energy is transmitted or received through the ceramic package in one or more radiation lobes of the antenna. Another example is in the field of microwave wireless LANs in which microwave communication between electronic devices may take place through a transmissive surface. A further example is an antenna system in which an antenna is protected by a radome, and wherein EM energy is transmitted or received by the antenna through the radome. A radome is physical covering for protecting an enclosed antenna from adverse environmental effects such as electrical and structural degradation caused by extremes of moisture temperature, pressure and vibration. In the case of aircraft-mounted antenna systems, a radome also provides protection against aerodynamic stress, kinematic heating and birdstrike.
Transmission or reception of radiation by an antenna through a transmissive surface frequently gives rise to adverse effects on one or more characteristics of the radiation for certain parameters, or combinations of parameters, of the transmitted or received radiation such as antenna lobe position and direction with respect to the surface, frequency, and polarisation. For example, for certain combinations of antenna lobe position and direction, standing waves may be set up between the antenna and the surface when the antenna is transmitting at a certain frequency, resulting in reduced transmission efficiency of the radiation. The amplitude of the radiation is therefore reduced as it passes through the transmissive surface. The presence of the transmissive surface also generally gives rise to a deviation in the direction of transmitted or received radiation on passing through the surface. For example in the case of an antenna system having an antenna provided with a radome, this effect results in boresight error, i.e. a difference in the direction in which radiation leaves the antenna and the direction in which it propagates after passing through the radome.
Under certain conditions, a portion of the radiation transmitted by the antenna may be transmitted along the surface. In addition to representing an additional transmission loss mechanism this can have other undesirable effects. For example in the case of an antenna system comprising an antenna provided with a radome, EM energy may be conducted along the surface of the radome to the radome's fixing points from where EM energy is radiated into a large solid angle, potentially interfering with other antenna systems nearby.
In the case of high-bandwidth signals, diffraction and dispersion effects at the surface may give rise to an undesirable angular dispersion of a transmitted or received radiation on passing through the surface. Certain parameters, or combinations of parameters, of the transmitted or received radiation may also result in its polarisation being affected in an undesirable way.
Thus one or more characteristics of transmitted or received EM radiation, such as angular extent, direction, amplitude and polarisation may be affected on passing through the surface.
It is an object of the invention to ameliorate one or more of these problems.